David Peat

Quantum Uncertainty 9that its nuclei are unstable and spontaneously break apart or “decay”into the element radon. Physicists knew that after 1,620 years only halfof this original radium will be left—this is known as its half-life. Aftera further 1,620 years only a quarter will remain, and so on. But anindividual atom’s moment of decay is pure chance—it could decay in aday, or still be around after 10,000 years.The result bears similarity to life insurance. Insurers can computethe average life expectancy of 60-year-old men who do not smoke ordrink, but they have no idea when any particular 60-year-old will die.Yet there is one very significant difference. Even if a 60-year-old doesnot know the hour of his death, it is certain that his death will be theresult of a particular cause—a heart attack, a traffic accident, or a boltof lightning. In the case of radioactive disintegration, however, there isno cause. There is no law of nature that determines when such an eventwill take place. Quantum chance is absolute.To take another example, chance rules the game of roulette. Theball hits the spinning wheel and is buffeted this way and that until itfinally comes to rest on a particular number. While we can’t predict theexact outcome, we do know that at every moment there is a specificcause, a mechanical impact, that knocks the ball forward. But becausethe system is too complex to take into account all the factors involved—the speed of the ball, the speed of the wheel, the precise angle at whichthe ball hits the wheel, and so on—the laws of chance dominate thegame. As with life insurance, chance is another way of saying that thesystem is too complex for us to describe. In this case chance is a measureof our ignorance.Things are quite different in the quantum world. Quantum chanceis not a measure of ignorance but an inherent property. No amount ofadditional knowledge will ever allow science to predict the instant aparticular atom decays because nothing is “causing” this decay, at leastin the familiar sense of something being pushed, pulled, attracted, orrepelled.Chance in quantum theory is absolute and irreducible. Knowingmore about the atom will never eliminate this element. Chance lies atthe heart of the quantum universe. This was the first great stumbling